The present invention is directed to using laser bonding to connect two electrical members together. In particular, the present invention is advantageous in inner lead bonding of a tape automated bonding (TAB) tape to the electrical bumps on an integrated circuit die, such as semiconductors. Thermal compression bonding, the current industry standard for inner lead bonding, uses 15,000 psi pressures and 400 degrees C. temperatures which would have a damaging effect if leads were bonded to bonding pads or bumps coated over semiconductor structures, especially as the bonding pads become more miniaturized. Further, thermal compression bonding is commonly optimized to one specific integrated circuit type and different sized circuits require a different set of tooling which is time consuming to replace and re-optimize. In addition the thermode must be cleaned periodically resulting in decreased system throughput. Thermalsonic bonding is also well known, but suffers the drawback that the ultrasonic energy can damage the materials, the speed is limited, and the resolution may not be sufficient for closely pitched bonds.
It is well known that highly reflective metals are difficult to laser bond since laser irradiation occurs in the visible and near-visible regions of the spectrum. For CO2 lasers, this reflectivity is traditionally overcome by coating the metals to be joined with an organic material, such as flux, that absorbs the energy and transfers the heat via thermal conduction. The use of additional coating that must be removed after bonding is undesirable, particularly on devices that have close pitches which makes thorough cleaning difficult. Failure to clean residual organics can degrade the reliability of the circuit. Several solutions have been proposed. U.S. Pat. No. 4,023,005 discloses a method of welding highly reflective metallic members wherein one member is coated with a metal skin of nickel or palladium with low reflectivity to enable welding by a laser. The metal skin is chosen which will not vaporize and as the molten alloy of the metallic members cools a weld nugget alloy is formed containing metal from the metal skin as well as the metallic members. Likewise, U.S. Pat. No. 4,697,061 discloses a method for laser welding a highly reflective covering to a base layer wherein both the covering and the base layer are covered with a metal skin of solder that is less highly reflective of the laser. Each of these prior art techniques, however, suffers the drawback that intermetallics containing the low reflectivity metal and the highly reflective metals occur at the bond interface. At this location these intermetallics can cause reliability problems, especially if the bond is subject to thermal cycling, thermal shock, or mechanical shock. In the case of copper/gold bonds, a tin coating may form brittle intermetallic compounds throughout the entire bond interface.